A crucial design problem for replacement steam generators at the San Onofre nuclear plant was recognized by the equipment manufacturer and plant operator, but the design team decided against making changes because it would require further regulatory approval, says a report released Friday.

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The documents provide the most detailed glimpse to date into the vetting process for the design of generators that failed as soon as a year after installation and have left San Onofre offline for 13 months.

The report, written by manufacturer Mitsubishi Heavy Industries, was delivered to nuclear safety regulators in October 2012 but was kept from public view because of proprietary information. Extensive portions were redacted by Mitsubishi before release.

The design team staffed by Mitsubishi and plant operator Southern California Edison considered at least three modifications to address steam flows but decided against them in part because of regulatory provisions that can trigger a reassessment of overall plant safety, the report indicates.

In a written response Friday, Edison said it never rejected a proposed design change to address the steam flow problems based on regulatory compliance issues.

San Onofre Chief Nuclear Officer Pete Dietrich said Edison was never advised by Mitsubishi that the steam flow issues could contribute to the failure of generator tubes.

“At the time, the design was considered sound,” Dietrich said in the statement.

Sen. Barbara Boxer, who last month flagged the Mitsubishi report and urged its release, said the portions made public Friday support concerns that Edison and Mitsubishi rejected safety modifications to avoid triggering a more rigorous license amendment review process by the Nuclear Regulatory Commission.

The report shows the equipment manufacturer went to great lengths to brace tubes in areas of turbulent steam. Instead, the tubes ended up vibrating in parallel with the braces and hitting each other in unprecedented fashion. A radiation release on Jan. 31, 2012, shut down the plant and led to the discovery of rapid tube degradation of generator tubing.

“In their zeal to prevent one kind of vibration they kind of caused the other,” said Murray Jennex, who worked as a systems engineer at San Onofre for nearly 20 years and now teaches at San Diego State University. “They didn’t pay attention as closely to in-plane vibration and that’s what got them.”

Steam generator tubes are of special safety concern because they act as a barrier to radioactive water and are a component of the reactor cooling system.

The replacement generators added hundreds of tubes to compensate for a new tube alloy, but the generators were required to fit within the same space as the originals to avoid major changes to plant specifications.

Entire pages of the 65-page report were blanked out by Mitsubishi, including several passages devoted to the “contributing causes” of generator faults. Boxer described the redactions “significant.”

A lengthy technical supplement to the report also was released, with redactions.

“There is really nothing wrong — in fact it is a best practice with safety at top of mind — when you tell your manufacturer let’s not disrupt the safety standards of the plant, how the plant is run,” Manfre said.

Edison insists that it eventually received assurances from Mitsubishi that the generators were safe, including a warranty.

“MHI assured us that the generators were safe and sound based on their calculations,” said Manfre of Edison.

Mitsubishi’s report focuses on its own accomplishments and shortfallings in the design and manufacture of the steam generators.

Edison continues to blame computer codes used in the design phase for misstating generator steam flows. Mitsubishi asserted in its report that the use of more widely accepted computer models would not have identified inadequate margins for damaging steam flows.

Edison is at odds with Mitsubishi over provisions of a 20 year warranty that limits the manufacturers liability to $138 million. The tab for replacing the generators so far exceeds $670 million, with a final accounting due next week from Edison, the plant’s majority owner.

The Nuclear Regulatory Commission’s Office of Investigations has been conducting since September an investigation into the completeness and accuracy of information provided by Edison about the generators. Commission spokesman Victor Dricks said Friday that the investigation is still under way.

Nuclear commission staff based in Texas and Southern California are evaluating Edison’s proposal to restart one of the plant’s reactors at partial power for a short period of time to see if it can dampen destructive vibrations among steam generator tubes.

Damon Moglen, of the nuclear safety group Friends of the Earth, urged regulators to put off the restart at least until the investigation is complete into information provided by Edison.

“This seems to suggest that Edison was aware all the way back in 2005 that there were problems and that rather than making changes that would have triggered a slow down and an amendment and a license review they chose to go ahead,” Moglen said.

Friends already has petitions pending before the nuclear commission accusing Edison of violating regulations by avoiding a more thorough review of the steam generators and seeking a semi-judicial license amendment review of the restart plan.

The contents of the Mitsubishi report also could hold implications in the California Public Utilities Commission investigation into whether utility customers are due a bill refund or suspension costs associated with the sidelined plant.